This invention relates to an electrostatographic printing machine, and more particularly concerns an apparatus for fusing colored images.
In a typical electrostatographic printing machine, a latent image is recorded on a surface and developed with charged particles. After the latent image is developed, a sheet of support material is positioned closely adjacent thereto so as to receive the particles therefrom. The particles are then permanently affixed to the sheet of support material forming a copy of the original document thereon. Electrographic and electrophotographic printing are differing versions of electrostatographic printing. The process of electrophotographic printing employs a photoconductive member arranged to be charged to a substantially uniform level. The charged photoconductive member is exposed to a light image of an original document. The light image irradiates the charged photoconductive member dissipating the charge in accordance with the intensity of the light transmitted thereto. This records an electrostatic latent image on the photoconductive surface. Electrographic printing differs from electrophotographic printing in that neither a photoconductive member nor a light image of the original document are required to create a latent image on the surface. Both of the foregoing processes generally employ heat settable particles to develop the latent image. The particles are commonly fused to the sheet of support material by the application of heat and pressure thereto.
Various techniques have been developed for applying heat to the particles on the sheet of support material. One technique is to pass the sheet of support material with the powder image thereon through a pair of opposed rollers. In one such system, a heated fuser roll and a non-heated backup roll are employed.
In the most commonly employed type of heated roll fuser, the heated fuser roll has the outer surface thereof covered with a polytetrafluoroethylene commonly known as Teflon to which a release agent such as silicone oil is applied. The Teflon layer, preferably, has a thickness of about several mils.
More recently fuser systems have been utilizing silicone rubber and Viton (Trademark of E. I. dupont) coated fuser rolls for contacting the toner images to thereby enhance copy quality, that is to say, perceived copy quality.
Bare roll fusers while not commercially accepted have been making inroads, at least in the patent literature.
Heretofore, no single roll fuser like those discussed above or for that matter no device utilizing a single fuser structure of any type has been produced which satisfactorily fuses colored images on various substrates.
The problem of fusing colored images is addressed in U.S. Pat. No. 4,223,203 wherein there is disclosed a heat and pressure fusing apparatus for fixing toner images to copy substrates comprising a first fusing system consisting of a pair of nip forming rolls, one of which is provided with a conformable outer surface and a second fusing system consisting of a pair of nip forming rolls, one of which has a rigid outer surface. Copy substrates are passed sequentially through the nips of the first and second fusing systems, in that order such that the toner images sequentially contact the conformable outer surface and then the rigid outer surface.
The fusing of color images has been accomplished commercially in a copier wherein a radiant fuser for opaque copies and a post vapor fusing process for transparencies are employed. Such a system is expensive as is the two fuser arrangement of the U.S. Pat. No. 4,223,203. Additionally, the radiant and post vapor arrangement is not capable of producing glossy color copies.
While not specifically addressing the problems of fusing colored images, other patents such as U.S. Pat. No. 3,965,331 utilize multiple fuser structures. The U.S. Pat No. 3,965,331 discloses a contact fuser assembly for use in an electrostatic reproducing apparatus wherein toner images are formed on various types of substrates, for example, plain paper and transparency materials such as cellulose acetate or polyester film. The fuser assembly is characterized by a provision of three fuser rolls forming a pair of nips through which the substrates pass in order to fuse the toner images thereto. Transport mechanism is provided for conveying the substrates to one or the other of the nips depending upon the particular material of the substrate. The surface of the roll provided for contacting the plain paper comprises a hard metal surface and the roll for contacting the toner images carried by the cellulose acetate, etc. comprises an elastomeric surface.
Other patents which contain teachings that may be relevant to the instant invention are U.S. Pat. Nos. 3,566,076; 3,679,302; 3,861,863; 4,526,459; 4,627,813 and 4,639,405 the former which relates to a method and apparatus for fixing toner images in which a copy sheet bearing unfixed toner is first passed through a pair of heated fuser rollers and is subsequently passed through surfacing rollers to provide a gloss to the toner image. In order to prevent curling of the copy sheet and blistering of the glossed image, the copy sheet is passed through a conditioner means, located between the fuser rollers and the surfacing rollers, for removing a substantial portion of the moisture from the copy sheet.
U.S. Pat. No. 4,627,813 discloses a thermal fixing apparatus for use with a copying machine or electronic printer in which an operating temperature of the apparatus after energization is quickly reached. A pair of fixing rolls is provided, at least one of which is heated. The outer surface of the other is covered with an elastically deformable outer layer. The two rolls are pressed into abutment with one another to form a nip therebetween of the predetermined width. A plate-shaped heater element is disposed prior to the nip adjacent the path of conveyance of toner-image-bearing paper sheets to be fixed. The plane of the heater element is preferably parallel to the plane of the paper. The surface temperature of the heater element has a temperature higher at central portions than at widthwise ends thereof to provide uniform fixing conditions.
U.S. Pat. No. 3,861,863 discloses a black and white image fuser comprising a first stage backside heater and a second stage soft roll fuser.
U.S. Pat. No. 3,679,302 discloses first and second stage radiant fusers.
U.S. Pat. No. 3,566,076 discloses the combination of radiant and pressure fusing.
U.S. Pat. No. 4,526,459 discloses a heat and pressure fuser including a heated fuser roll and two pressure rolls which cooperate to form two nips. Copy substrates are passed through one or the other of the two nips depending on the path of travel of the copy substrate. Thus, when the substrate moves along a first path it passes through one of the nips and when it moves along a second path it passes through the other of the nips.
For a color copier to find total acceptance in the market place it is necessary that it be able to reproduce glossy or matt opaque copies and high chroma transparencies. Moreover, it is necessary that it be able to do so without increasing the cost of the copying machine to the customer and without utilizing excessive space in the machine. Chroma refers to the quality of the projected images by the transparency. In other words, the colors of a projected transparency represent a faithful reproduction of the colors of the original images.